The invention is directed to thermoplastic films and more particularly to a process for making films having controlled surface finish.
A process of calendering a sheet or a film having at least one textured surface entailing passing a web of polymeric resin through a nip between a rigid roll and a resilient roll is disclosed. The inventive process comprises slowing down the cooling rate of the web by rolling its textured surface about the resilient roll. The rate of cooling is in comparison to a corresponding process where the web is rolled about said rigid roll. Products thus produced are suitable for making thermoformed articles having low gloss.
In the extrusion and subsequent calendering of synthetic thermoplastic films (in the present context, the term xe2x80x9cfilmxe2x80x9d refers to sheets and to films) it is frequently desirable to use at least one roll which has a relatively resilient surface in order to maintain relatively uniform pressures across the width of the sheet material and to compensate for variation in the thickness of the sheet passing into the nip between a pair of cooperating rolls. While in some instances, two resiliently faced rolls may be employed, generally most applications use a combination of one resilient roll with one rigid roll such as a steel roll. Such combinations are often employed for preparing sheets, or films having embossed surfaces and/or polished surfaces. As used herein, the term xe2x80x9ctexturedxe2x80x9d, in reference to the surface of a sheet, includes textured surfaces and surfaces exhibiting high gloss.
It has long been recognized that when films having textured surfaces are subjected to subsequent heating, such as in the course of thermoforming, they often warp or lose at least some of their texture. Matted surfaces often become undesirably glossy.
The present invention is directed to an improvement in the process for making films having textured surfaces. Textured films produced in accordance with the present invention feature greater surface stability. The process of thermoforming, or, for that matter, any subsequent thermal treatment, would not easily destroy the surface characteristics which has been imparted to the sheet by calendering.